Conventionally, there is known a shift control system for an automatic transmission as described, for example, in a gazette JP-A 2-14933. This gazette describes a technique for determining the engagement pressure of engagement-side friction elements based on the throttle opening. With this, the engagement pressure is determined with respect to driver's acceleration/deceleration requirement, achieving smooth shifting. Moreover, in the prior art as disclosed in a gazette JP-A 5-99004, it is described a technique for determining the engagement pressure with the throttle opening fixed during inertia phase.
Now, an explanation is made about requirements to be achieved when carrying out torque-reduction control during upshift with the engine being in the normally driving state, i.e. in the drive state so called.
(Requirement 1) First, shift-control start determination is made so that the engagement-side oil pressure starts to increase. When the engagement-side friction elements start to transmit torque, torque phase is started to decrease output-shaft torque. Then, when transmission torque to the engagement-side friction elements which increases continuously exceeds to torque corresponding to input torque, the inertial phase so called is started where input rotation is increased. In the inertia phase, engine rotation and the like are lowered, so that inertia constituents are released into output-shaft torque, resulting in an increase in output-shaft torque. As for the shape of output-shaft torque at this time, in terms of physical feel, the difference is preferably minimized between output-shaft torque when torque is reduced maximally in torque phase and output-shaft torque after starting the inertia phase.
(Requirement 2) Likewise, as for output-shaft torque at completion of the inertia phase, in terms of physical feel, the difference is preferably minimized between output-shaft torque immediately before completing the inertia phase, which is determined by the height of the engagement-side shelf pressure, and output-shaft torque immediately after completing the inertia phase, which is determined by input torque and the gear ratio at completion of the inertia phase.
(Requirement 3) Output-shaft torque during inertia phase is preferably varied in accordance with the throttle opening corresponding to a driver's intention. This means that if the throttle opening is constant, driver's acceleration/deceleration requirement is not provided, so that output-shaft torque is preferably as nearly constant as possible within the range of failing to interfere with other tasks, whereas in the throttle opening is varied, driver's acceleration/deceleration requirement is provided, output-shaft torque is preferably varied in accordance with variations in engine torque which varies in correspondence with that requirement.
Using the technique described in the gazette JP-A 2-14933, for example, to meet the above requirements, the requirement 3 can be satisfied. However, if the engine speed is different, input torque after completing the inertia phase is different even with the same throttle opening. This renders impossible control of the relationship with output-shaft torque immediately before completing the inertia phase, which is determined by the shelf pressure determined in accordance with the throttle opening, so that the requirements 1 and 2 cannot be satisfied.
Further, when supposing determination of the engagement pressure based on input torque, the requirements 1 and 2 can be satisfied. However, turbine rotation is decreased during inertia phase, in accordance with which input torque is increased. Thus, even when the throttle opening is constant, and the driver has no intention to carry out acceleration or deceleration, the engagement-side oil pressure is increased, which results in an increase in output-shaft torque, leading to impossible achievement of an object that “if the throttle is constant, output-shaft torque is preferably as nearly constant as possible within the range of failing to interfere with other tasks”.
Furthermore, using the technique described in the gazette JP-A 5-99004,the throttle opening is kept constant, so that the requirement 3 cannot be satisfied in like manner.